q.I
have seen a device called a storm glass which consists of a glass tube
filled with a liquid containing crystals. The shape of the crystals is
supposed to change according to the pressure. How does this device work?
Apparently, Darwin used one on board the Beagle.

SOREN WIUM-ANDERSEN
Hillerod
Denmark

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a. The
storm glass is often found as part of an Admiral Fitzroy's barometer,
examples of which are fairly common in antique shops and can be bought
for a few hundred pounds. Robert Fitzroy was captain of the Beagle
during its most famous voyage, so it is likely that Darwin would have
used the storm glass.

A letter published in School Science Review (June 1987) suggested
that the storm glass was invented by an alchemist for Italian sailors.
The author, P. J. Towse, also suggested schools could make a storm glass
by mixing 10 grams of camphor, 40 cubic centimetres of ethanol, 2·5
grams of potassium nitrate, 2·5 grams of ammonium chloride and 33 cubic
centimetres of distilled water, then placing the mixture in a corked
test tube.

Changes in the liquid are supposed to show how the weather will change.
Clear liquid means bright weather; dim liquid means rain. If it is dim
with small stars, expect thunderstorms. Large flakes mean it will be
overcast or, in winter, snowy. Crystals at the bottom mean frost and
threads in the upper part mean it will be windy. If the liquid contains
small dots, expect humid or foggy conditions, and if it contains small
stars on sunny winter days, expect snow in a few days.

Presumably some of the observations can be accounted for by changes in
solubility--as the ambient temperature and pressure change--and by the
varying speed at which this happens. But some of it, I suspect, is pure
alchemy.

PETER BORROWS
Epping
Essex

a. About
forty years ago I found a recipe for the liquid in a storm glass. I
prepared a sample, sealed it in a glass tube and left it on the roof of
a factory in south London for six months. I examined the tube every day
and tried to correlate atmospheric conditions with changes in the
crystals in the tube. I could find no correlation, but as the tube was
sealed, its contents could only react to temperature changes and not
pressure.

B WHEELER
Sandton
South Africa

a.
I send this letter to you with some urgency. It concerns the answer to
the question about the storm glass. There is the possibility that
somebody may be seriously hurt trying to make this device without having
first thought about what they are going to do.

Camphor is a pungent organic chemical, which is combustible. It has a
history of use as a plasticiser in explosives and is readily oxidisable;
ammonium chloride decomposes when heated, releasing toxic fumes and
potassium nitrate is an oxidising agent. Mixing these three chemicals
together in a dry state could very probably result in the potassium
nitrate oxidising the camphor, causing it to combust and the ammonium
chloride to decompose. The consequences may be fatal. If this device
really must be made, probably the safest way to do this is to dissolve
the potassium nitrate and ammonium chloride in the water, then add the
ethanol and finally the camphor.

ADMIRAL FITZROY'S REMARKS ON THE CAMPHOR GLASS

(taken
from "The Weather Book" circa 1850)

Having often noticed peculiar
effects on certain instruments, used as weather glasses, that did not seem to be
caused by pressure, or solely by temperature, by dryness, or by moisture -
having found that these alterations happened with electric changes in the
atmosphere that were not always preceded or accompanied by movement of mercury
in a barometer, and that, among other peculiarities, increase or diminution of
winds, in the very 'heart' of the trades, caused effects on them, while the
mercurial column remained unaltered, or showed only the slight inter-tropical
diurnal change (as regular there as a clock), we have long felt sure that
another agent might be traced.

Considerably more than a century ago
what were called 'storm glasses' were made in this country. Who was the
inventor, is now very uncertain; but they were sold on old London Bridge, at the
sign of the "Looking Glass".

Since 1825 we have generally had
some of the vials, as curiosities rather than otherwise, for nothing certain
could be made of their variations until lately, when it was fairly demonstrated
that if fixed, undisturbed, in free air, not exposed to radiation, fire or sun,
but in the ordinary light of a well-ventilated room, or, preferably, in the
outer air, the chemical mixture in a so-called storm glass varies in character
with the direction of the wind - not its force, specially, though it may so vary
(in appearance only) from another cause, electrical tension.

As the atmospheric current veers
towards, comes from, or is only approaching from the polar direction, this
chemical mixture - if closely, even microscopically watched, - is found to grow
like fir, yew, or fern leaves - or like hoar frost - or even large but delicate
crystallisations.

As the wind, or great body of air,
tends more from the opposite quarter, the lines or spikes - all the regular,
hard, or crisp features, gradually soften and diminish till they vanish.

Before and in a continued southerly
wind the mixture sinks slowly downwrd in the vial, till it becomes shapeless,
like melting white sugar.

Before or during the continuance of
a northerly wind (polar current), the crystallisations are beautiful (if the
mixture is correct, the glass a fixture, and duly placed); but the least motion
of the liquid disturbs them.

When the main air-currents meet, and
turn towards the west, making easterly winds, stars are more or less numerous,
and the liquid dull, or less clear. When, and while they combine by the west,
making westerly wind, the liquid is clear, and the crystallisation well defined,
without loose stars.

While any hard or crisp features are
visible below, above, or at the top of the liquid (where they form for polar
wind) there is plus electricity in the air; a mixture of polar current co-exisiting
in that locality with the opposite, or southerly.

When nothing but soft, melting,
sugary substance is seen, the atmospheric current (feeble or strong as it may
be) is southerly with minus electricity, unmixed with and uninfluenced by the
contrary wind.

Repeated trials with a delicate
galvanometer, applied to measure electric tension in the air, have proved these
facts, which are now found useful for aiding, with the barometer and
thermometers, in forecasting weather.

Temperature affects the mixture
much, but not solely; as many comparisons of winter with summer changes of
temperature have fully demonstrated.

A confused appearance of the
mixture, with flaky spots, or stars, in motion, and less clearness of the
liquid, indicates south-easterly wind, probably strong - to a gale.

Clearness of the liquid, with more
or less perfect crystallisations, accompanies a combination, or a contest, of
the main currents, by the west, and very remarkable these differences are - the
results of these air currents acting on each other from eastward, or entirely
from an opposite direction, the west.

The glass should be wiped clean, now
and then, - and two or three times in a year the mixture should be disturbed, by
inverting and gently shaking the glass vial.

The composition is camphor - nitrate
of potassium and sal-ammoniac - partly dissolved by alcohol, with water, and
some air, in hermetically sealed glass.

There are many imitations, more or
less incorrectly made.

Those camphor glasses used by the
writer lately were prepared by Messrs. Negretti and Zambra. There are numerous
others, some of which are inexact in chemical composition; and are not nearly so
sensitive.